Thermal disconnect couplings have been utilized in a variety of applications for the purpose of protecting system components from an overheated condition by responding to some predetermined high temperature to operate to disconnect driven components of the system from the source of motive power. A typical application is in the electrical power generation system for an aircraft. In such a system, a generator is ultimately powered by an aircraft engine, frequently via a constant speed drive. The thermal disconnect coupling is typically disposed between the engine and the constant speed drive and is provided with a coolant-lubricant such as hydraulic oil from the constant speed drive. When the coolant-lubricant temperature exceeds some predetermined value, representative of an overheated condition, the coupling acts to break the driving connection between the engine and the constant speed drive.
Most frequently, such thermal disconnect couplings include a driving shaft connected to the engine and a driven shaft connected to a constant speed drive. The two shafts are coupled to each other by interengaging teeth. One of the shafts, most frequently the driven shaft, is movable relative to the driving shaft so as to allow the teeth to disengage to break the driving connection. Movement from the engaged to disengaged position is restrained in normal operation by a body of eutectic material such as solder, having a predetermined melting point that corresponds with the maximum permitted temperature of the lubricant-coolant in the system. Consequently, in normal operation, the body of eutectic material will be in the solid phase and prevent relative movement between the shafts that would result in a breaking of the driving connection. However, when the high temperature limit is exceeded, the body of eutectic material will change to the liquid phase and is then permitted to flow to a location other than its original one, releasing the normally restrained shaft for movement relative to the other to break the driving connection.
The very fact that systems utilize thermal disconnect couplings for system protection makes it implicit that system components are intended to be reused once the connection is re-established after suitable maintenance. In some prior art thermal disconnect couplings, the maintenance chore preliminary to reuse has been complicated by the fact that flow of the eutectic material is not restrained. Consequently, it may enter coolant flow passages in the surrounding area to impede coolant flow or may even flow to bearings to subsequently solidify, ultimately requiring replacement of such bearings as part of the maintenance preliminary to reuse. At the very least, maintenance preliminary to reuse requires the time-consuming removal of such eutectic material that has flowed to the location of interfitting components. One prior art construction that may be subject to such difficulty is illustrated in U.S. Pat. No. 4,086,991 issued May 2, 1978 to Swadley.
In order to avoid these difficulties, the prior art has suggested that the body of eutectic material be contained in a single, sealed chamber having a substantially larger volume than that occupied by the body of eutectic material. Consequently, when the body of eutectic material changes to the liquid phase, it may flow sufficiently within such chamber so as to permit relative movement between the shafts resulting in the decoupling of the same, and yet, be retained within the chamber so as to avoid flow to other parts of the system.
However, because the eutectic material of which such bodies is typically formed is relatively soft, and the typical coupling of this type places the body of eutectic material under compression, the same has a tendency to cold flow, which in turn can result in a disconnection of the drive under normal operating conditions.
To avoid this problem, the prior art has resorted to the placement of relatively stiff wires in the body of eutectic material which are intended to be relatively rigid in compression so long as the body of eutectic material is in the solid phase. Thus, the resulting composite structure can be made sufficiently resistant to cold flow as to prevent inadvertent breaking of the driving connection. A representative example of this type of construction is found in U.S. Pat. No. 4,271,947 issued June 9, 1981 to Gaeckle.
The difficulties with this type of coupling are twofold. Firstly, the forming of the body of eutectic materials with properly oriented wires is a more complicated procedure than would be called for where the body of eutectic material essentially fills a chamber and thus is not subject to cold flowing.
A second difficulty resides in the fact that the chamber, in which the eutectic material is placed, must be of considerably greater volume than that of the eutectic material employed in order to allow the material to flow sufficiently that the relative movement between the shafts may take place to result in disconnection. This in turn increases the overall size of the coupling. The increased size of the coupling frequently is a considerable disadvantage, particularly in aircraft applications where size and weight constraints virtually always exist.
The present invention is directed to overcoming one or more of the above problems.